Modems transmit and receive digital data to and from a network system, such as the general switch telephone network (GSTN). Typically, a network will communicate a string of digital data for every oscillation of its internal clock crystal. A string of digital data often represents a "symbol," and the number of symbols communicated in one second is referred to as the "symbol rate." It should be noted that a string of digital data may be a single bit or multiple bits of data information. Many modems demodulate data at a symbol rate different from that of the network's symbol rate and different from that of the symbol rates of other modems. Therefore, most modem designs include techniques for interpolating or decimating data from one symbol rate into the symbol rate used by the receiving modem. When demodulating information from a remote modem with a different symbol rate, it is desirable for the receiving modem to generate a clock waveform having the same frequency, and hence symbol rate, as that of the remote modem.
Conventional modems generate a clock waveform through a hardware device capable of receiving a supply signal and offsetting the supply signal to generate a clock waveform. In most conventional modem designs, this waveform generator is supplied by a signal within the receiving modem that has already been interpolated or decimated to correct for the timing differences between the remote modem and the receiving modem. Therefore, the signal supplying the waveform generator contains the necessary information for the waveform generator to match the frequency of the remote modem. However, it is desirable for the waveform generator to be supplied from an independent source rather than from a signal within the receiving modem. As used herein, an "independent source" is any source other than the receiving modem that generates a supply signal for the waveform generator. For example, it is desirable for the waveform generator to be supplied by a signal directly from a network in communication with the receiving modem. However, since a signal from such an independent source has not already been interpolated or decimated to correct for the timing differences between the receiving modem and the remote modem, a supply waveform from the independent source contains no information indicating how much the waveform generator should offset the supply signal in order to match the clock frequency of the remote modem.
Thus, a heretofore unaddressed need exists in the industry for providing a system and method for regenerating a synchronous clock waveform in a receiving modem using a supply waveform from an independent source.